Bonding of lead wires to a microcircuit chip mounted on a lead frame for coupling to external circuitry is generally accomplished by ball bonding. According to this technique, a lead wire or bonding wire is held in a capillary tool so that the lead wire projects beyond the end of the tool. The capillary tool forms part of a ball bonding machine in which the tool is appropriately mounted and positioned over the metalized die pad of an integrated circuit chip. A ball is formed at the end of the lead wire or bonding wire by, for example, arc discharge between the bonding wire and another electrode. After solidification, the metal ball at the end of the lead wire is brought into intimate contact with the metalized die pad and a bond is formed typically by thermocompression ultrasonic bonding.
Prior art methods of ball formation for bonding lead wires generally suffer from lack of control over the ball formation process with consequent variation in the size, shape, and oxidation of the bonding ball. Further disadvantages of prior art ball bonding machines and methods are discussed in the "Background Art" section of U.S. Pat. No. 4,390,771.
According to the improved apparatus and method for bonding wire ball formation disclosed in U.S. Pat. No. 4,390,771, a ball is formed at the end of bonding wire extending from a capillary tool by substantially enclosing the end of the bonding wire in a shroud or shield, flooding the shroud or shield and the end of the bonding wire with an inert gas, and generating a controlled electrical arc discharge between the lead wire and the shroud or shield which function as complementary electrodes. The method according to U.S. Pat. No. 262,595 permits concise control and metering the energy for melting and forming a ball of uniform quality. In particular, according to the method disclosed, a controlled pulse train of electrical pulses is generated and delivered for arc discharge between the wire and shroud so that the quantity of energy delivered by the pulse train may precisely match the energy requirements for melting a ball of desired dimensions.